const express = require("express");
const app = express();

// middleware
app.use((req, res, next) => {
  console.log("Middleware chal gaya");
  next(); // next middleware / route ko bhejta hai
});

app.get("/", (req, res) => {
  res.send("Hello World");
});

app.listen(3000);


📌 Types of Middleware
1️⃣ Application level

app.use(myMiddleware)



2️ Route level
app.get("/admin", myMiddleware, (req,res)=>{
  res.send("Admin page");
});



3️⃣ Built-in

Jaise:

app.use(express.json());




Error middleware
app.use((err, req, res, next) => {
  res.send("Error aaya");
});



👉 app.use(express.json()) kya karta hai?

Ye line Express ko bolti hai:

“Agar koi request JSON format me aaye, to usko read karke JavaScript object bana do aur req.body me daal do.”


Maan lo Postman ya frontend se tum ye bhejte ho:

{
  "name": "Rahul",
  "age": 22
}


Agar tumne likha hai:

app.use(express.json());


to Express automatically:

✅ JSON ko parse karega
✅ JS object banayega
✅ req.body me daal dega

Phir tum controller me likh sakte ho:


app.post("/user", (req, res) => {
  console.log(req.body);
});


Output:

{ name: "Rahul", age: 22 }



Agar express.json() NA likho?

To:

❌ req.body = undefined
kyonki Express ko pata hi nahi ki JSON kaise read karna hai.


One-line me:

👉 app.use(express.json()) = JSON translator




👉 express.urlencoded() kya karta hai?

Ye middleware HTML form se aane wala data read karta hai aur usko bhi req.body me daal deta hai.

Matlab:


Real example (HTML form):
<form action="/login" method="POST">
  <input name="email" />
  <input name="password" />
  <button>Submit</button>
</form>


Browser actually backend ko aise bhejta hai:

email=test@gmail.com&password=123456


⚠️ Ye JSON nahi hota — isko bolte hain URL encoded data.


Ab ye line ka magic:
app.use(express.urlencoded({ extended: true }));




Express ko bol rahe ho:

👉 “Is type ke encoded form data ko samajh ke req.body me daal do.”

Phir route me:

app.post("/login", (req, res) => {
  console.log(req.body);
});


Output:

{
  email: "test@gmail.com",
  password: "123456"
}


❓ { extended: true } ka matlab?

Simple version:

✅ extended: true

Nested / complex objects allow karta hai:

<input name="user[name]" />
<input name="user[age]" />


Becomes:

{
  user: {
    name: "Rahul",
    age: "22"
  }
}



👉 express.static() kya karta hai?

Ye Express ko bolta hai:

“Is folder ke andar jo files hain (CSS, images, JS, etc.), unko directly browser ko de dena.”

Matlab frontend assets serve karta hai.



Example:
app.use(express.static("public"));


Iska matlab:

👉 public folder ko publicly accessible bana diya.


project/
│
├─ app.js
└─ public/
   ├─ style.css
   ├─ logo.png
   └─ main.js


   Ab browser me directly access kar sakte ho:
CSS:
http://localhost:3000/style.css

Image:
http://localhost:3000/logo.png

Short recap:

express.json() → JSON read

express.urlencoded() → Form data read

express.static() → CSS / images / JS serve